1. Field of the Invention
Embodiments of the present disclosure generally relate to power conversion and, more particularly, to a method and apparatus for improved burst mode operation.
2. Description of the Related Art
Solar panels, or photovoltaic (PV) modules, convert energy from sunlight received into direct current (DC). The PV modules cannot store the electrical energy they produce, so the energy must either be distributed to an energy storage system, such as a battery or pumped hydroelectricity storage, or distributed by a load. One option to use the energy produced is to employ one or more inverters to convert the DC current into an alternating current (AC) and couple the AC current to the commercial power grid. The power produced by such a distributed generation (DG) system can then be sold to the commercial power company.
PV modules have a nonlinear relationship between the current (I) and voltage (V) that they produce. A maximum power point (MPP) on an I-V curve for a PV module identifies the optimal operating point of the PV module; when operating at this point, the PV module generates the maximum possible output power for a given temperature and solar irradiance. Therefore, in order to optimize power drawn from a PV module, a power conversion device coupled to the PV module, such as an inverter or a micro-inverter, generally employs a maximum power point tracking (MPPT) technique to ensure that the PV module is operated at the current and voltage levels corresponding to its MPP. The MPPT acts to rapidly adjust the PV module operating current and voltage levels in response to changes in solar irradiance and/or temperature such that the PV module can continue to operate at the MPP.
During the time period required for an MPPT technique to bias a PV module to its MPP, for example, when the solar irradiance on a PV module changes from no irradiance to increasing irradiance or at a PV module/inverter initial activation, a power conversion device coupled to the PV module will suffer from a lower efficiency until the MPP is achieved. Additionally, a power conversion device coupled to a PV module generally will suffer from a lower efficiency when the PV module is operating at a low power, e.g., low irradiance. During low irradiance, a PV module and an associated inverter may operate so inefficiently that is it best for overall system efficiency to deactivate the PV module and/or its inverter until solar irradiance increases.
In some instances, AC power from the power grid is subject to phase shift causing the voltage and current to be out of phase. The out-of-phase component of the AC power is commonly referred to as reactive power set by rule or request. The grid or PV plant operator may require that the inverters operate with a phase shift between the AC grid voltage and the AC current generated in order to help stabilize grid voltage, or to inject a desired amount of reactive power into the grid to maintain the mains voltage at a proper level in view of time inductance. Such a phase shift impacts the PV voltage and current and may degrade MPPT accuracy. The inverters knows what power factor/phase shift to operate with, where the phase shift Phi, such that tan(phi)=Q/P, where Q is the amount of reactive power generated (positive or negative) and P is the real generated power.
Therefore there is a need in the art for a method and apparatus to achieve MPP with AC power having a reactive component.